This invention relates to a display tube comprising an evacuated envelope having substantially flat, parallel spaced-apart front and rear walls and a plurality of support means dividing the interior of the envelope into a plurality of modules extending between the front and rear walls for substantially the full height of the envelope and a cathodoluminescent screen on the interior of the front wall, each module having means for producing and directing an electron beam along one of a plurality of paths extending toward the screen, an electron multiplier extending substantially transverse to said paths for amplifying the electron beam, and deflection means for deflecting the electron beam in a direction transverse to the said paths.
Such a flat panel display tube is described in published British Patent Application No. 2110465. This display tube is suitable for providing a display area of around 0.75 to 1M.sup.2. The interior of the tube envelope is divided into a plurality of horizontally adjacent modules by the support walls, which extend vertically and contact and support the front wall, that is the faceplate. In consequence, the faceplate can be of a thickness substantially thinner, and therefore lighter, than for a conventional cathode ray tube faceplate. In a display tube embodiment described in this published application each module has an electron gun which produces a beam of electrons and directs the beam of electrons along a first path substantially parallel to the rear wall of the envelope, and deflecting electrodes carried on the rear wall, comprising a plurality of parallel, spaced-apart electrodes extending transverse to the electron beam's first path, which are selectively operable to deflect by electrostatic forces the electron beam from that first path through substantially ninety degrees into one of a plurality of second paths extending towards the electron multiplier and the screen.
By providing an electron multiplier in each module it is possible to use a low voltage, low current beam to effect frame scanning vertically of the module, this scanning being accomplished by the deflecting electrodes on the rear wall with the beam being deflected from its first path sequentially through the plurality of second paths. This means that the beam current can be kept sufficiently low to avoid the effects of space charge blow-up of the electron beam. Also, low voltages can be used by the deflecting electrodes. Thereafter the electron beam is amplified by the electron multiplier to provide a high current beam which is accelerated towards the screen by high voltages applied via electrodes on the support means defining the margins of the module.
Line scanning, widthwise of the module, is accomplished by way of the deflection means which deflect the electron beam transversely of the plurality of paths. These deflection means are constituted by pairs of parallel electrodes which are disposed between the electron multiplier and the screen on the supports, and which for example extend substantially perpendicular to the screen and heightwise of the module.
In comparison with conventional display tubes offering similar sized display areas, the aforementioned display tube is considerably smaller both in weight and overall dimensions, particularly its depth. However, whilst the electron guns are arranged in their respective modules to direct the electron beam produced thereby substantially parallel to the rear wall of the envelope so as to allow a reduction in the depth of the display tube to some extent, sufficient space must still be provided in this region of each module to allow the electron beam to be deflected from its first path through substantially ninety degrees by the deflecting electrodes towards the electron multiplier.
A modular kind of cathode ray display tube having some similarities with the above described tube is disclosed in British Patent Specification No. 2127616. In an embodiment described in FIGS. 6 and 7 of this specification, each module is provided with a vertically arranged area emitter as a source of electrons and a planar array of discrete, vertically-spaced electrodes disposed between the area emitter and the electron multiplier each having one aperture therethrough corresponding to a line of the display to be produced. The apertures are arranged in a row and define on the side of the electrode array remote from the emitter a plurality of vertically-spaced beam paths. The electrodes are individually addressed so as to prevent or allow electrons to pass through the aperture in the electrode and by addressing the electrodes in sequence, electrons are allowed to pass through the aperture of each successive electrode in turn to produce an electron beam following only one of the plurality of paths at any one time. In this way frame scanning is achieved with the apertures determining respective lines of the display. Line scanning is accomplished by deflection electrodes located adjacent the multiplier output.
Whilst provision of the electron emitter and switching electrode arrangement in this manner enables an electron beam to be defined along the plurality of paths easily and conveniently and requires less space so that the volume of this region of the display tube, in particular the distance between the rear wall of the envelope and the electron multiplier is reduced significantly, the arrangement suffers from the disadvantage that driving the tube is made more complicated in view of the need to address each of the apertures in a module individually. For a tube having say, a 500 line display, 500 apertured electrodes will be required, each being interconnected to a switchable driving circuit.